1. Field of the Invention
This invention relates generally to disk drives, and more particularly to a magnetic recording disk drive that includes a system for canceling the effects of rotational vibration.
2. Description of the Related Art
Magnetic recording hard disk drives (HDDs) use an actuator, typically a rotary voice-coil-motor (VCM) type of actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data.
Disk drives experience rotational vibration and disturbance forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system.
Rotational vibration (RV) cancellation is a method that uses sensors (typically accelerometers) to detect rotational vibration and improve the PES by canceling the off-track motion induced by the rotational vibration. The RV sensor signal is input to a feedforward controller that creates a feedforward compensation signal that is summed with the control signal to the VCM actuator. The use of a RV sensor and feedforward compensation in this manner is well-known, as described by Jinzenji et al., “Acceleration Feedforward Control Against Rotational Disturbance in Hard Disk Drives,” IEEE Transactions on Magnetics, Vol. 37, No. 2, March 2001, pp. 888-893; and M. T. White et al., “Increased Disturbance Rejection in Magnetic Disk Drives by Acceleration Feedforward Control,” Proceedings of the 13th Triennial IFAC World Congress, Jun. 30-Jul. 5, 1996, San Francisco, Calif., pp. 489-494.
The RV cancellation performance is dependent on the accuracy of gain and phase matching of the RV sensor signal to the VCM actuator. The VCM gain, i.e., its output or response to a control signal input, is not fixed at its design or optimal value, but varies with both temperature and position of the head on the disk. Typically the VCM gain can vary by up to about 20% from its optimal value.
The two conventional implementations of acceleration feedforward control for RV cancellation are analog and digital. The analog implementation uses an analog circuit to generate an analog feedforward compensation signal that is summed with the analog VCM control signal. The analog approach has the advantage of good RV cancellation because there is no phase delay. However, the analog circuit, which is designed so that the RV sensor gain matches the design or optimal VCM gain, is not able to adjust for changes to the VCM gain. The digital implementation uses a digital processor to generate a digital VCM control signal, which requires analog-to-digital conversion and digital sampling of the RV sensor signal, which in turn cause a phase delay. The processor can implement a digital filter to recover the phase delay, but the phase lead filter increases the gain at high frequency, so the gain matching becomes worse at high frequency.
What is needed is a disk drive with a method for RV cancellation that uses the advantages of both the analog and digital implementations while minimizing their disadvantages.